1893.] Mineralogy and Petrography. 1093 
One of the micas? in the Mte. Doré trachyte is an anomite with an 
optical angle of 41°. When treated with boiling hydrochloric acid it 
loses its greenish color, with the extraction of its iron and magnesium, 
and becomes less strongly doubly refracting. After an hour's treat- 
ment it becomes colorless and uniaxial, when its optical sign is nega- 
tive. 
Having examined seventy-one uniaxial minerals with respect to their 
heat conductivity, Jannetaz™ finds that only five contradict his law 
that the major axis of the isothermal ellipsoid is parallel to the direc- 
tion of the principal cleavage, and the minor axis normal thereto. 
Instruments.—Laspeyres” describes a modification of the setting 
of the condenser above the polarizer of the microscope, that enables the 
observer to change rapidly from converged to parallel light, even when 
an object is being examined. The lower nicol is in its usual position. 
The condenser is imbedded in a metal strip set into the stage, and 
sliding easily in a groove prepared for it. 
A goniometer with two circles, enabling the operator to measure 
nearly all the planes on a crystal with one adjustment of the latter, is 
explained in detail by Goldschmidt,” who also illustrates its use by 
several examples. 
% Bull. Soc. Franc d. Min., XV, 1892, p. 97. 
* Ib. X V, p. 133. 
55 Zeits. J. Kryst., XXI, p. 256. 
TIN. p. 210. 
